If you're familiar with electronics: Imagine you have a current source, driving 1 amp through a variable resistor set to a very low resistance. It only takes less than a volt to sustain that current, so your current source is quite happy to do this. Then, without turning off the current source, you increase the resistance: now the current source needs a lot more voltage to sustain that current.
From the current source's point of view, the voltage across the resistor looks indistinguishable from a voltage source pushing back against it. Even though that voltage is coming from the resistive load, even though that voltage only exists across the resistor because of the 1 amp that the current source is itself driving, the load acts the same as a voltage source fighting to drive current backwards into the current source.
Imagine that your current source is not ideal: it has voltage limits. If you want to squeeze the most power out of your current source, you'll set the resistance up such that the resulting output voltage is near the limit of what your current source can handle while still supplying 1 amp. If you increase the resistance further, you'll exceed the voltage rating and possibly damage your current source. Then even if you return the resistance to a low level after that, you might not get 1 amp anymore from that damaged source.
All of this has been an analogy. Permanent magnets are a lot like non-ideal sources that cannot turn off. To squeeze the very most out of the magnet, you want to configure the load such that it's driving the magnet to its limits. When you remove the rotor from the stator, it now has to push magnetic field through air, rather than through steel. This increases the effective demagnetizing load working against the magnet (known as "reluctance", analagous to resistance). It's no different from the magnet's point of view than if you'd looped an electromagnet wire around it fighting against its magnetization. Permanent magnets are magnetized through a hysteresis process, and with a strong enough demagnetizing field, the internal domains can flip and the magnet gets demagnetized.
From the current source's point of view, the voltage across the resistor looks indistinguishable from a voltage source pushing back against it. Even though that voltage is coming from the resistive load, even though that voltage only exists across the resistor because of the 1 amp that the current source is itself driving, the load acts the same as a voltage source fighting to drive current backwards into the current source.
Imagine that your current source is not ideal: it has voltage limits. If you want to squeeze the most power out of your current source, you'll set the resistance up such that the resulting output voltage is near the limit of what your current source can handle while still supplying 1 amp. If you increase the resistance further, you'll exceed the voltage rating and possibly damage your current source. Then even if you return the resistance to a low level after that, you might not get 1 amp anymore from that damaged source.
All of this has been an analogy. Permanent magnets are a lot like non-ideal sources that cannot turn off. To squeeze the very most out of the magnet, you want to configure the load such that it's driving the magnet to its limits. When you remove the rotor from the stator, it now has to push magnetic field through air, rather than through steel. This increases the effective demagnetizing load working against the magnet (known as "reluctance", analagous to resistance). It's no different from the magnet's point of view than if you'd looped an electromagnet wire around it fighting against its magnetization. Permanent magnets are magnetized through a hysteresis process, and with a strong enough demagnetizing field, the internal domains can flip and the magnet gets demagnetized.